1. Field of the Invention
The present invention relates to a thermal printer utilized in printers, facsimile equipments or the like, and particularly to a thermal printer capable of reproducing halftone image on image receiving paper.
2. Description of the Prior Art
In a conventional thermal printer capable of reproducing halftone image such method has been adopted which comprises steps of dividing each element unit of image to be reproduced into a plurality of dots, and suitably selecting ratio of black dots and white dots in this element unit, whereby tone graduation, i.e., light and shade, of the image is simulatively reproduced.
In such prior art, however, it is required that one element unit of the image is composed of a large number of dots in order to express various grade of tone graduation, and further it is necessary for providing a plurality of thermal heads corresponding to each one element unit of image, and yet there is such a disadvantage that resolution is not very high.
In another conventional thermal printer for effecting the halftone reproduction in image, attention is paid to characteristics of image receiving paper on which image is reproduced. FIG. 1 shows a relationship between energy to be impressed to a thermal head and density of dots on the image receiving paper wherein a range indicated by .DELTA.E has favorable linearity. Attention is paid to such range, and the energy to be impressed to the thermal head is controlled within the aforesaid range, thus tring to achieve halftone image reproduction.
In this method, however, the range which can be utilized is limited, and even in such range linearity is not completely insured. Thus, such thermal printer has a disadvantage in that errors take place in the halftone reproduction so that favorable image reproduction onto receiving paper cannot be obtained, besides contrast in the reproduced image is insufficient.